Load-carrying skeletal members, such as the human hip, frequently are rendered non-functional because of fracture, damage, disease, resections for malignancy or disease, or because of pain or malformation. Such members are commonly repaired by total joint replacement with artificial components, and one type of bone replacement that has been particularly successful over the past 22 years is that of the human hip. Such hip prostheses typically include a femoral portion or component which is implanted in the femur and an acetabular component which is secured to the pelvis. The femoral component includes a head which rotates in a socket formed in the acetabular component. Examples of such prostheses are illustrated in U.S. Pat. Nos. 3,744,061; 4,012,796; 4,146,936; 4,156,943; 3,808,606; 3,102,536; and 4,080,666.
Many of these known prosthetic devices require the use of cement for embedment of the femoral component into the hollow bone structure. While many known cemented systems usually provide uniform distribution of mechanical loads, elimination of relative motion between the prostheses and the bone, and satisfactory load-per-unit area, many are also subject to problems associated with the toxicity of the cement, necrosis of the adjacent bone and incomplete filling of the cavity in the bone, and absence of resiliency. In addition, various reports of long-term results of cemented total hip replacements with a minimum follow-up of ten years show that the loosening rate on the femoral side is in the neighborhood of 30 to 50 percent. Specifically, Moreland (Moreland, J. R.; Gruen, T. A.; Mai, L.; and Amstutz, H. C.: "Aseptic Loosening of Total Hip Replacement: Incidence and Significance," In the Hip: Proceedings of the Eighth Open Scientific Meeting of The Hip Society, pp. 281-291, St. Louis, C. V. Mosby, 1980) reports a 44 percent loosening rate on the femoral side for periods of less than ten years; Salvatti, et al. (Salvatti E. A.; Wilson, P. D., Jr.; Jolley, M. N.; Vakili, F.; Agialli, P.; and Brown, G. C.: "A Ten Year Follow-up Study of Our First One Hundred Consecutive Charnley Total Hip Replacements", 63A J. Bone Joint Surg., 753-767, 1981.) indicates that the femoral components were loose in 33 percent of the cases studied; Mueller (Mueller, M. E. "Long Term Follow-up of Total Hip Replacements" presented at the AOA International Symposium on "Frontiers in Total Hip Replacement", May 1981, Boston Mass.) reports that loosening caused a reoperation rate of about 20 percent in ten years; and a Mayo Clinic report (Coventry, M. B.: "Ten Year Follow-up Study of Total Hip Replacement at the Mayo Clinic", presented at the AOA International Symposium on "Frontiers in Total Hip Replacement," Boston, Mass., May, 1981) indicates a 30 percent rate of failure in the fixation of the femoral component within ten years. Methylmethacrylate is used as a cement in many such implants, and it has been shown that a membrane with synovium-like characteristics forms at the junction of the cement and the bone as a biological response to methylmethacrylate. (Goldring, S.; Schiller, A.; and Harris, W. H.: "Synovial Like Transformation at the Cement Bone Interface Following Total Hip Replacement", presented at the AOA International Symposium on "Frontiers in Total Hip Replacement," May 1981, Boston. Mass.) This membrane is characterized by the enhanced capacity to generate prostaglandin E.sub.2 (PG E2) and collagenase, two of the key ingredients for bone destruction.
Because of the problems associated with earlier cemented implants, more recent developments in orthopedic research have been directed at generating implants which are fixed to the skeleton by bony ingrowth without the use of any cement. Examples of devices utilizing the bony ingrowth technique are illustrated in U.S. Pat. Nos. 3,314,420; 2,688,139; 3,808,606; and 3,938,198. Two major types of such implants are femoral surface replacements and intramedullary, stemmed femoral components. Experience has shown that the use of surface replacements on the femoral side for total hip replacement is not particularly successful. Failure rates of 27 percent and 39 percent have been reported by Capello (Capello, W.: "Results of Synovium Surface Replacement", presented at the AOA International Symposiumm on "Frontiers in Total Hip Replacement", May 1981, Boston, Mass.) and Freeman (Freeman, M. A. R. "Results of I.C.L.H. Surface Replacement", presented at the AOA International Symposium on "Frontiers in Total Hip Replacement", May 1981, Boston, Mass.) with follow-up periods of about three years. The reasons for this high failure rate include osteoporosis from disuse, bone resorption from peak stresses, femoral neck fracture, lysis, avascular necrosis, and technical errors, and in certain patients with severe disease, the lack of sufficient bone stock. As a result stemmed implants are generally considered to be most desirable, and the majority of the devices used, are stemmed components. Although some of the above stemmed implants are satisfactory for most purposes, none of them employing bony ingrowth fixation allows for easy removal of the implant should that become necessary because of metal fatigue, or other problems.